Nitrogen oxide (NOx) gases, such as nitric oxide (NO) and nitrogen dioxide (NO2), are pollutants that may be produced when fuel is combusted at high temperatures in internal combustion engines. These gases may have adverse health effects, and may participate in the formation of smog and acid rain. In order to comply with increasingly demanding low NOx emission regulations, engine manufacturers may be compelled to use technologies that substantially decrease NOx emissions from engine exhaust. One such technology is selective catalytic reduction (SCR) aftertreatment systems which catalyze the reduction of NOx in exhaust gas to nitrogen and water prior to release of the exhaust gas from an exhaust outlet, such as a tailpipe. In a SCR aftertreatment system, a reducing agent is injected as a liquid into the exhaust gas stream of the exhaust pipe, and the mixture of the reducing agent and the exhaust gas is passed through a downstream SCR catalyst which uses the reducing agent to catalyze the reduction of NOx in the exhaust gas stream. The reducing agent may be ammonia, or it may be urea that is subsequently hydrolyzed to ammonia in the exhaust gas stream. In the context of diesel engines, a reducing agent consisting of urea and water is referred to as diesel exhaust fluid (DEF).
The reducing agent should be evaporated and well mixed with the exhaust gas prior to introduction to the SCR catalyst to ensure that the reduction of NOx at the SCR catalyst proceeds efficiently. Evaporation of the reducing agent not only assists even distribution of the reducing agent in the exhaust gas, but also avoids undesirable accumulation of reducing agent deposits in the exhaust pipe that could lead to decreased conversion efficiencies as well as increased back pressure in the exhaust pipe. To promote evaporation of the reducing agent and mixing of the reducing agent with the exhaust gas, a mixer may be provided in the exhaust pipe between the injector and the SCR catalyst. However, it may be a challenge to provide a mixer that meets performance standards in terms of both mixing the reducing agent with the exhaust gas, and curtailing deposit accumulation by promoting reducing agent evaporation.
One example of a reducing agent mixer is described in U.S. Pat. No. 8,607,555. The patent discloses a mixing element that includes a grid supporting rows of trapezoidal deflector elements that are oriented in different directions. The patent also discloses a mixing element that includes four fields of deflector elements that are turned 90° with respect to each other to generate rotational motion to the exhaust gases and reducing agent flowing through the mixer.
Although the above mixing elements are effective, there is still a need for improved mixing systems that promote both evaporation of the reducing agent and mixing of the reducing agent with the exhaust gas prior to introduction to the SCR catalyst.